The present invention relates to an oxygen production process of VSA (vacuum swing adsorption) type comprising a periodic or exceptional regeneration.
The production of oxygen from atmospheric air by units of PSA (pressure swing adsorption) type has undergone a significant expansion in recent decades. Improvements have been made to the adsorbents, the technology and the process itself.
Generally, the terms PSA are used to designate any gas purification or separation process implementing a cyclical pressure swing experienced by the adsorbent between a high pressure, called adsorption pressure, and a low pressure, called regeneration pressure. Thus, this generic designation of PSA is employed indifferently to designate the following cyclic processes, to which it is also common practice to give more specific names based on the pressure levels involved or on the time needed for an adsorber to return to its initial point (cycle time):                The VSA processes in which the adsorption is performed substantially at atmospheric pressure, preferentially between 0.95 and 1.25 bar abs and the desorption pressure is lower than atmospheric pressure, typically from 50 to 400 mbar abs.        The MPSA or VPSA processes in which the adsorption is performed at a high pressure higher than atmospheric pressure, typically between 1.35 and 6 bar abs, and the desorption at a low pressure lower than atmospheric pressure, generally between 200 and 650 mbar abs.        The actual PSA processes in which the high pressure is substantially higher than atmospheric pressure, typically between 3 and 50 bar abs and the low pressure substantially equal to or higher than atmospheric pressure, generally between 1 and 9 bar abs.        The RPSA (rapid PSA) processes for which the duration of the pressure cycle is typically less than one minute.        The URPSA (ultra rapid PSA) processes for which the duration of the pressure cycle is of the order of at most a few seconds.With the above definitions, the present invention relates equally to the VSA and VPSA processes.        
In order to simplify the text, only the term VSA will be employed to encompass the scope of the invention as has just been defined. It will be recalled that it concerns more specifically a VSA O2 unit producing oxygen (generally 85 to 95% molar and more often than not 90 to 93% mol.).
Whatever the type of PSA, an adsorber will begin an adsorption period until it is loaded into the constituent or constituents to be stopped at the high pressure then will be regenerated by depressurization and extraction of the adsorbed compounds before being reconditioned, in practice repressurized, to recommence a new adsorption period. The adsorber has then performed a “pressure cycle” and the very principle of the PSA process is to string together these cycles one after the other; it is therefore a cyclic process. The time that an adsorber takes to revert to its initial state is called cycle time (Tc). In principle, each adsorber follows the same cycle with a time offset that is called time phase (Tp) or more simply phase. A PSA process therefore involves N volumes of adsorbent following the same cycle and offset in time by Tp=Tc/N.
There are a large number of possible cycles for the VSA O2 units that it is pointless to list here.
The most commonplace industrial cycles comprise 1 to 4 adsorbers, that is to say from 1 to 4 unitary volumes of adsorbent.
Unlike many processes, in the case of oxygen production, the raw material, that is to say the atmospheric air, is free and the energy consumption of the unit is one of the important, even predominant items in the oxygen production cost, particularly in the case of a high production, for example greater than 120 tons/day, in which, because of the scale effect, the weight of the investment rounded to the nearest Nm3 of oxygen produced is lesser.
Because of this, the lesser gain on the specific energy is advantageous because it directly and substantially affects the production costs.
One of the basic trends is therefore to use adsorbents increasingly specific to N2/O2 separation, that is to say exhibiting the best N2/O2 selectivity combination, N2 capacity, kinetic energy . . . and to possibly manage the thermal effects to approximate an optimal isothermal operation.
Since these specific adsorbents have a high cost compared to the more basic adsorbents (such as a zeolite of calcium A type for example), another trend for limiting the investment and thereby the overall cost of production of the oxygen is naturally to reduce the quantity needed thereof by using in particular increasingly shorter cycles.
This trend began several years ago and one of the major problems was very soon identified: the less adsorbent that is used, the greater the risk of pollution to which is added the fact that the more specific zeolites that have just been mentioned are, per contra, increasingly sensitive to this pollution. It should actually be noted that the adsorbent volume has decreased much more than the quantity of air introduced per hour, for a given O2 production. The oxygen extraction efficiency—around 50/60%—has changed relatively little compared to the reduction of the volume of adsorbent.
It is therefore commonplace on the industrial VSA O2 units to change the adsorbent after 4 or 5 years of service to restore the initial performance levels. The zeolite is generally shipped to the supplier who reactivates it, reconditions it and makes it available once more to the industry. This load will generally be used to subsequently fill another new unit or a unit being cleaned.
It is enough to have a sufficient load of zeolite available in advance to perform the replacement in relatively short lead times, of the order of a week for example.
Nevertheless, such a procedure has a relatively high cost and presents risks. To the cost of downtime of a load of adsorbent, must be added the draining, storage, transportation, reactivation and filling. During handling operations on site and in the factory, 5 to 10% of the product is lost and has to be replaced with new adsorbent. Most of the risks correspond to the filling during which it is obviously necessary to avoid any entry of moisture. For that, effective means have to be implemented to avoid any prolonged contact between the adsorbent and the moist atmospheric air. Bad weather conditions are likely to delay these filling operations and result in cost overheads.
It is then easy to understand all the interest that there is in finding another way of maintaining, over time, the initial—and maximal—performance levels of a VSA O2 unit, without having to perform the periodic change of large quantity of adsorbent or without needing to install costly regeneration systems associated with sophisticated dimensionings of large adsorbers capable of withstanding high thermal stresses.
Starting from that, one problem which arises is how to provide an improved VSA O2 process.